Stability and Seaworthiness

A yacht's stability depends on the relationship between two forces: the centre of gravity (G) — the point through which the total weight acts downward — and the centre of buoyancy (B) — the centre of the underwater volume through which buoyancy acts upward. When the boat heels, B moves to leeward as the underwater hull shape changes, creating a righting moment that tries to return the boat upright. The distance between the vertical lines through G and B is the righting lever (GZ).

Several factors reduce stability and must be carefully managed. Carrying heavy gear aloft (for example a dinghy on the coachroof or heavy items in a deck locker) raises the centre of gravity, reducing the righting moment. Free surface effect occurs when liquids — such as water in the bilge or partially filled tanks — slosh to the low side as the boat heels, effectively raising G and reducing stability. Flooding through an open hatch or hull breach has the same effect and is compounded by the added weight.

Keeping the boat seaworthy means reducing topweight wherever possible: stow heavy items low in the bilge, avoid carrying unnecessary weight on deck, and secure all loose gear. Ensure hatches, washboards, and ventilators can be sealed in heavy weather. A well-maintained hull with properly functioning bilge pumps, seacocks, and skin fittings is fundamental to seaworthiness. Before any passage, check that all through-hull fittings have accessible seacocks and that softwood bungs are tied to each seacock in case a hose fails.